Nitrite, an Electron Donor for Anoxygenic Photosynthesis
Benjamin M. Griffin,* Joachim Schott, Bernhard Schink
A
lthough compounds of the sulfur cycle, and more recently the iron cycle, are well studied electron donors for anoxygenic photosynthesis, no analogous oxidations in the ni trogen cycle are known. We report a previously unknown process in which anoxygenic photo trophic bacteria use nitrite as an electron donor for photosynthesis, providing a microbial mechanism for the stoichiometric oxidation of nitrite to nitrate in the absence of oxygen. To examine nitrite as a possible electron donor for anoxygenic photo trophs, we established enrichment cultures derivedfrom local sewage sludge and several freshwater sediments in anoxic, bicarbonate buffered mineral medium (1). Low amounts of nitrite (1 to 2 mM) were fed repeatedly to avoid toxicity, and the cultures were incubated continuously in the light.
After incubating in the light for several weeks, enrichment cultures from 10 out of 14 sampling sites oxidized nitrite to nitrate and de veloped pink coloration, as typical of anoxygenic phototrophs. Absorption spectra of intact cells revealed maxima at 799 nm and 854 nm, which are characteristic of bacteriochlorophyll a (2). No chlorophyll a or oxygen was observed in nitrite
oxidizing cultures, suggesting that nitrate did not form because of a combination of oxygenic photosynthesis and aerobic nitrification. No growth or nitrite oxidation occurred in cultures incubated in the dark or in uninoculated bottles, thereby ruling out the possibilities that nitrate was produced by anaerobic ammonia oxidation (anammox) or abiotic, photochemical processes.
Light dark shift experiments performed over several days with enrichment cultures transferred five times showed that growth and nitrate produc tion depended on both light and nitrite (Fig. 1).
The rate of nitrite consumption increased on mul tiple feedings and approached 2 mM per day after 1 week in the light. As expected for a photoauto trophic process, nitrite consumed, nitrate produced, and biomass formed were all tightly correlated; ni trate was formed from nitrite near stoichiometrically.
We isolated the numerically dominant coccus (2 to 3mm in diameter) from the most active en richment culture derived from Konstanz sewage sludge by dilution to extinction in liquid medium (Fig. 1C) (1). Analysis of the 16S ribosomal RNA gene sequence revealed that the strain, des ignated KS, is most closely related toThiocapsa
roseopersicina(98% identical).Thiocapsaspe cies are widely distributed purple sulfur bacteria of the order Chromatiales and are metabolic gen eralists capable of photoautotrophic growth on a variety of common inorganic electron donors, in ad dition to aerobic chemolithoautotrophic growth (3).
Although phototrophs are known to directly influence the nitrogen cycle through reductive processes such as nitrogen fixation, assimilation, and respiration (4), this is the only example of a photosynthetically driven oxidation in the nitro gen cycle. In principle, this photosynthetic pro cess could compete for nitrite in the environment with other key nitrogen cycle processes such as denitrification, aerobic nitrification, or anammox.
In 1970, Olson proposed in detail how the water oxidizing activity of oxygenic photosynthe sis may have evolved from anoxygenic photo synthesis through a series of inorganic nitrogen electron donors with increasing midpoint potentials (5). The nitrite nitrate couple, with a standard redox potential of +0.43 V, could theo retically donate electrons to the quinone type reaction center in pur ple sulfur bacteria, where the bac teriochlorophyll primary donor has a midpoint potential as high as +0.49 V (6). This work demon strates nitrite as the highest potential electron donor for anoxygenic photo synthesis known so far and provides a modern example of an electron donor once implicated in the evo lution of oxygenic photosynthesis.
References and Notes 1. Materials and methods are available
onScienceOnline.
2. J. F. Imhoff, inAnoxygenic Photosynthetic Bacteria, R. E.
Blankenship, M. T. Madigan, C. E.
Bauer, Eds. (Kluwer, Dordrecht, Netherlands, 1995), pp. 1 15.
3. J. F. Imhoff, inThe Prokaryotes, M. Dworkinet al., Eds. (Springer Verlag, New York, ed. 3, 2006), vol. 6, pp. 846 873.
4. J. P. Megonigal, M. E. Hines, P. T.
Visscher, inTreatise on Geochemistry, vol. 8, W. H. Schlesinger, Ed.
(Elsevier, Amsterdam, 2003), pp. 317 424.
5. J. M. Olson,Science168, 438 (1970).
6. M. A. Cusanovich, R. G. Bartsch, M. D. Kamen, Biochim. Biophys. Acta153, 397 (1968).
Supporting Online Material
www.sciencemag.org/cgi/content/full/316/5833/1870/DC1 Materials and Methods
References
3 January 2007; accepted 19 April 2007 10.1126/science.1139478
Department for Biology, Universität Konstanz, D 78457 Konstanz, Germany.
*Present address: Institute for Genomic Biology, University of Illinois, Urbana, IL 61801, USA. To whom correspondence should be addressed. E mail: griff113@uiuc.edu
0 2 4 6 8 10
0 2 4 6 8
Time (d)
Nitrate or Nitrite (mM)
A 0.6 B
0 1 2 3
0 2 4 6 8
Time (d)
Nitrate or Nitrite (mM)
0.00 0.04 0.08 0.12
OD660
- +
Fig. 1. Time courses for nitrite consumed (
▲
), nitrate C produced (■
), cumulative nitrite consumed (♦), and growth as the change in optical density (DOD660) (●
) for triplicate enrichment cultures (N = 3). Data are mean ± SD. (A) Initially incubated in the light. (B) Initially incubated in the dark. The plus signs indicate nitrite feedings, and arrows denote a switch from the initial light condition. The minus signs indicate when the cultures were starved of nitrite to assess nitrite dependence of growth. (C) Phase-contrast micrograph of strain KS. The scale bar represents 10mm.1870
First publ. in: Science 316 (2007), p. 1870
Konstanzer Online-Publikations-System (KOPS) - URL: http://www.ub.uni-konstanz.de/kops/volltexte/2008/5950/
URN: http://nbn-resolving.de/urn:nbn:de:bsz:352-opus-59504
